Successful saves should be rare

For me, the way Elsewhere handles saves achieves two design goals.

PC's Saves

In order for danger to be effective in the game, two conditions have to be met.

First, the existence of the danger must be clear in advance, so the players can do something to avoid it. Realistic statues of people made of stone with dreadful expressions hint at the presence of a medusa, while a scorched wall and a skeleton can suggest a fire trap. Signaling the danger is the job of the GM. Good GMs know how to clearly indicate the danger without being implicit. This aspect belongs to the art of GM-ing and is not part of the system.

Second, the danger must be real. It must have the potential to kill or harm the PCs. Players should fear a snake's bite, a blade trap, or a hostile spellcaster.

On one hand, if saves were easy, players would rely on luck rather than thinking about how to bypass the danger. Therefore, we want successful saves to be very hard to achieve. In other words, we want a situation where you were affected by the danger because you (the player) were not careful enough, not because you were unlucky (although we will allow a small chance for you to save even if you were not careful).

On the other hand, resilient characters may be part of certain character tropes. If a player wants to invest resources to build such a highly resilient character (especially at high levels), we want to allow them to do so.

Representing saves as skills (Acrobatics, Athletics, Endurance, Judgment, Will) allows us to achieve both goals. Most characters will be untrained in the save-related skills. Since the DCs of the saves are usually high, their chances of saving by luck are slim. Players who invest in those skills will drastically increase their chances but will never become invulnerable, so the danger will still matter.

Enemies Saves

When a player casts a spell or performs a special stunt, they don't want it to fail due to bad luck alone (see here for an excellent analysis on 'output randomness'). Therefore, unless the enemy's trope suggests that they are resistant, we want them to fail the saves almost always. Unless the enemy has a high ability score or is trained in the relevant skill, we want the enemy to be affected. The GM can either decide to ignore the save and let them fail automatically or roll the save (with a slim chance) to add some excitement from time to time.

PCs' spell power (and thus the save DC) increases quickly. Combining it with the untrained skill level enemies have makes the PC's spells reliable when successfully cast.